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A prominent dayside auroral event, occurred during an IMF BY-dominatedtime interval, and characterized by the contemporaneous longitudinal andlatitudinal cusp bifurcations, is reported. The event was recorded the 19 December 2002,between about 09:30–10:45 UT, by the ITACA2 twin auroral monitorssystem, in the Greenland-Svalbard zone. The splitting of the ionosphericfootprint of the geomagnetic cusp, traced by the dayside auroral activity,was recently identified with the signatures of antiparallel reconnectionepisodes ongoing at different magnetopause locations, during large IMFBY periods. The first part of the event showed a broad longitudinalbifurcation of the red-dominated cusp aurora, displaced in the prenoon andpostnoon, with a separation up to ~1800 km, during northeast directedIMF (clock-angle rotating from 45° to 90°). This observation widensthe range of IMF regimes that are known to drive a longitudinal bifurcationof the cusp, since previous case-studies reported these events to occurduring southeast/southwest oriented IMF (clock-angle ≈135°).The second part of the event, developed when the IMF turned to a nearlyhorizontal orientation (BY>>0, BZ~0, clock-angle~90°), and exhibited the simultaneous activation of the cuspauroras in three distinct areas: i) two of them associated to theabove-mentioned longitudinally bifurcated cusp (~73°–75° CGMlatitude, type 1 cusp aurora), and linked to (near)antiparallel magneticreconnection patches equatorward the northern and the southern cusp, ii) theother one characterized by isolated high-latitude (~76°–77°CGM latitude, type 2 cusp aurora) rayed arc(s) with intense green emission, andtriggered by (near)antiparallel merging at the northern lobe (usuallyobserved during positive IMF BZ), poleward the local cusp. During thisphase, the longitudinal separation of the low-latitude type~1 cusp aurora wasabout 1000 km wide, with a 500 km gap, while the latitudinal separationbetween low- (type 1) and high-latitude (type 2) cusp auroras, in the postnoon, wasabout 270–280 km at its maximum. The longitudinal gap, corresponding to azone with weak auroral emission, was found to likely map to the componentreconnection region at the subsolar magnetopause. The magnetic mergingtopology that can be drawn on the basis of the reported cusp auroras supportthe idea of a "mixed" merging scheme, with (near)antiparallel reconnectionat high-latitudes, and component reconnection in the subsolar region, asrecently proposed by other authors

annales geophysicae

Antiparallel magnetic merging signatures during IMF &amp;lt;i&amp;gt;B&amp;lt;sub&amp;gt;Y&amp;lt;/sub&amp;gt;&amp;lt;/i&amp;gt;&amp;gt;&amp;gt;0: longitudinal and latitudinal cusp aurora bifurcations